Prevent widgets sometimes appearing to move relative to each other (#7710)

Sometimes when moving a window, having a tooltip attached to the mouse
pointer, or scrolling a `ScrollArea`, you would see this disturbing
effect:


![drift-bug](https://github.com/user-attachments/assets/013a5f49-ee02-417c-8441-1e1a0369e8bd)

This is caused by us rounding many visual elements (lines, rectangles,
text, …) to physical pixels in order to keep them sharp. If the
window/tooltip itself is not rounded to a physical pixel, then you can
get this behavior.

So from now on the position of all
areas/windows/tooltips/popups/ScrollArea gets rounded to the closes
pixel.

* Unlocked by https://github.com/emilk/egui/pull/7709

crates/egui/src/containers/area.rs

@@ -553,13 +553,14 @@ impl Area {
             move_response
         };
 
+        state.set_left_top_pos(round_area_position(
+            ctx,
             if constrain {
-            state.set_left_top_pos(
-                Context::constrain_window_rect_to_area(state.rect(), constrain_rect).min,
-            );
-        }
-
-        state.set_left_top_pos(state.left_top_pos());
+                Context::constrain_window_rect_to_area(state.rect(), constrain_rect).min
+            } else {
+                state.left_top_pos()
+            },
+        ));
 
         // Update response with possibly moved/constrained rect:
         move_response.rect = state.rect();
@@ -580,6 +581,16 @@ impl Area {
     }
 }
 
+fn round_area_position(ctx: &Context, pos: Pos2) -> Pos2 {
+    // We round a lot of rendering to pixels, so we round the whole
+    // area positions to pixels too, so avoid widgets appearing to float
+    // around independently of each other when the area is dragged.
+    // But just in case pixels_per_point is irrational,
+    // we then also round to ui coordinates:
+
+    pos.round_to_pixels(ctx.pixels_per_point()).round_ui()
+}
+
 impl Prepared {
     pub(crate) fn state(&self) -> &AreaState {
         &self.state

crates/egui/src/containers/scroll_area.rs

@@ -2,6 +2,8 @@
 
 use std::ops::{Add, AddAssign, BitOr, BitOrAssign};
 
+use emath::GuiRounding as _;
+
 use crate::{
     Context, CursorIcon, Id, NumExt as _, Pos2, Rangef, Rect, Response, Sense, Ui, UiBuilder,
     UiKind, UiStackInfo, Vec2, Vec2b, emath, epaint, lerp, pass_state, pos2, remap, remap_clamp,
@@ -748,6 +750,12 @@ impl ScrollArea {
         }
 
         let content_max_rect = Rect::from_min_size(inner_rect.min - state.offset, content_max_size);
+
+        // Round to pixels to avoid widgets appearing to "float" when scrolling fractional amounts:
+        let content_max_rect = content_max_rect
+            .round_to_pixels(ui.pixels_per_point())
+            .round_ui();
+
         let mut content_ui = ui.new_child(
             UiBuilder::new()
                 .ui_stack_info(UiStackInfo::new(UiKind::ScrollArea))

crates/egui_demo_app/tests/snapshots/imageviewer.png

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